This invention relates to a method of using modems to measure impairment of communication lines.
Improvements have been made in modem designs which have expanded the range of usefulness of such equipment. Some of these improvements enable modems to monitor or measure certain characteristics of the communications line connected to the modem, such as the signal-to-noise ratio and phase jitter. One such monitoring system is described in the commonly assigned application Ser. No. 376,741 filed on May 10, 1982 now U.S. Pat. No. 4,516,216. Usually such systems are capable of analyzing the line while data is being transmitted, but without interfering with said data. The systems have been found to be very accurate in modems having automatic or adaptive equalization. However, these monitoring systems lose their effectiveness in modems which do not have automatic equalizers because they do not have means for adequately eliminating receiver intersymbol interference caused by linear distortions of the channel.